This invention relates to a vehicle air conditioning system including a refrigerant compressor and an electrically driven cooling fan, and more particularly to a control for adjusting the operation of the cooling fan so as to minimize the power consumption of the air conditioning system while maintaining adequate refrigerant cooling.
Most vehicle air conditioning systems include a refrigerant compressor that is mechanically driven by the vehicle engine through a selectively engageable clutch, and at least one electrically driven cooling fan that is selectively activated to provide supplemental air flow through a condenser for cooling and condensing compressed refrigerant. As with the compressor, the cooling fan behaves as an engine accessory load since the electrical energy for operating the cooling fan is ultimately generated by an engine-driven alternator or generator to prevent discharging of the vehicle storage battery. Thus, the power requirements of the compressor and the cooling fan both influence the engine fuel economy.
The primary purpose of the cooling fan is to move air through the condenser to augment removal of heat from the circulating refrigerant. However, increasing the flow of air through the condenser allows the same cooling performance to be achieved with a lower compressor capacity. In other words, there is a trade-off between the work performed by the cooling fan and the work performed by the compressor. However, in most cases, the trade-off is ignored and the cooling fan is activated either continuously or at a high duty cycle to ensure that there is always sufficient cooling of the refrigerant. While such a control philosophy is simple to implement, it fails to minimize the power consumption of the system and the associated accessory loading, which in turn, unnecessarily reduces the engine fuel economy. Accordingly, what is desired is a control that is also simple to implement, but that minimizes the system power consumption to improve engine fuel economy while maintaining adequate cooling of the condensed refrigerant.
The present invention is directed to an improved control method for a vehicle air conditioning system including a refrigerant compressor and a condenser cooling fan, wherein the power consumption of the condenser cooling fan is adjusted in a manner to minimize the combined power consumption of compressor and the condenser cooling fan while maintaining adequate cooling of the condensed refrigerant. When the air conditioning system is operating at less than full capacity, the combined power consumption of the compressor and the condenser cooling fan is determined for each of a series of operating intervals. The change in the combined power consumption from one operating interval to the next is computed to determine the effect of a prior cooling fan power adjustment, and the power level of the cooling fan is incrementally adjusted in the same direction as the prior adjustment so long as the prior adjustment resulted in at least a specified reduction of the combined power consumption. If the prior adjustment failed to produce the specified reduction in overall power consumption, the power level of the cooling fan is incrementally adjusted in the opposite direction so that the control continually seeks the cooling fan power level that will minimize the combined power consumption of the compressor and condenser cooling fan.